System and Method for Extending Gas Life in a Two Chamber Gas Discharge Laser System

ABSTRACT

A method and system for performing injects of halogen gas into the chambers of a two chamber gas discharge laser such as a MOPA excimer laser for allowing operation of the laser within acceptable parameters and compensating for ageing effects without the necessity of performing refills is described. A parameter reflecting efficiency of the laser is measured, and the change in the parameter with respect to the length of laser operation is estimated. The change in the parameter with respect to the pressure in one of the chambers is also measured. At a given time, the total change in the value of the parameter is estimated, and from this change in pressure that is needed to reverse the change in the value of the parameter is calculated. The pressure in the chamber is then changed to correct for the amount of time that the laser has been in operation.

FIELD OF THE INVENTION

The present invention relates generally to laser systems. Morespecifically, the invention relates to performing injects of reactivegas into the chambers of a gas discharge laser, such as a two chamberMaster Oscillator-Power Amplifier excimer laser.

BACKGROUND OF THE INVENTION

One type of gas discharge laser used in photolithography is known as anexcimer laser. An excimer laser typically uses a combination of a noblegas, such as argon, krypton, or xenon, and a reactive halogen gas suchas fluorine or chlorine. The excimer laser derives its name from thefact that under the appropriate conditions of electrical stimulation andhigh pressure, a pseudo-molecule called an excimer (or in the case ofnoble gas halides, an exciplex) is created, which can only exist in anenergized state and can give rise to laser light in the ultravioletrange.

Excimer lasers are widely used in high-resolution photolithographymachines, and are thus one of the critical technologies required formicroelectronic chip manufacturing. Current state-of-the-art lithographytools use deep ultraviolet (DUV) light from the KrF and ArF excimerlasers with nominal wavelengths of 248 and 193 nanometers respectively.

While excimer lasers may be built with a single chamber light source,the conflicting design demands for more power and reduced spectralbandwidth have meant a compromise in performance in such single chamberdesigns. One way of avoiding this design compromise and improvingperformance is by utilizing two chambers. This allows for separation ofthe functions of spectral bandwidth and pulse energy generation; eachchamber is optimized for one of the two performance parameters.

Such dual-gas-discharge-chamber excimer lasers are often called MasterOscillator-Power Amplifier, or “MOPA,” lasers. In addition to improvingthe spectral bandwidth and pulse energy, the efficiency of the dualchamber architecture can enable the consumable modules in MOPA lasers toreach longer operational lifetimes than their counterpart modules insingle chamber light sources.

In each chamber, as the light source discharges energy across itselectrodes to produce light, the halogen gas, fluorine in the case ofArF or KrF lasers, is depleted. This causes a decrease in the laserefficiency which is seen, for example, as an increase in dischargevoltage required to create a given desired pulse energy. Since thedischarge voltage has an upper limit determined by physical constraintsof the hardware, steps must be taken to replenish the lost fluorine sothat the voltage remains below this limit and the laser continues tofunction properly.

One way to do this is with a full replenishment of the gas in thechambers, called a refill, where all of the gas is replaced while thelaser is not firing to return the gas content in the chamber to thedesired mix, concentration and pressure. However, refills are extremelydisruptive as the laser must be shut off during the refill process, andthus the lithographic exposure of semiconductor wafers must also bepaused in a controlled manner at the same time and then restarted whenthe laser is again operational to avoid improper processing of thewafers. For this reason, it is typical to refill both chambers at onceto save time, although this is not necessary.

The need for a refill can depend on several complex and oftenunpredictable variables, including the light source firing pattern andenergy, the age of the light source modules, and others that will befamiliar to those of skill in the art. For this reason, refills aretypically done on a regular schedule, which ensures that the lightsource operation will never suffer unanticipated interruption due to thelight source reaching its operational limit. Such a regular schedulegenerally yields very conservative upper limits on the time betweenrefills, such that some users of the light source operating at low pulseusages might be able to wait for a much longer period of time betweenrefills than is provided by the simple schedule.

Given the demands of increased throughput and light source availability,efforts have been made to minimize light source stoppage for refills.One way of doing this is by performing a partial replenishment of thegas in the chambers, known as an inject, rather than a full refill. Aslong as the laser is able to continue to operate within certainparameters, it is not necessary to shut the laser down for the inject,and thus processing may continue during the inject process.

Another factor that decreases efficiency is the ageing of the laser;older excimer lasers are in general less efficient than newer ones.However, it can be difficult to separate the effects of ageing from theeffects of fluorine concentration that is not optimal, particularly whenthe control of the fluorine is not precise.

A number of prior methods and systems have been described for managinginjects, including, for example, how to determine when an inject shouldoccur and the amount of halogen gas to be provided by the inject. See,for example, U.S. Pat. Nos. 7,741,639 and 7,835,414, owned by theassignee of the present application. However, such prior art stillrequires that a refill be done at some point to keep the laser withinoperating parameters.

Further, many of these prior art techniques do not have adequate controlof the fluorine concentration to be able to tell whether a given loss ofefficiency is due to the fluorine level or to the ageing of the laser.As a result, those using such prior art techniques generally assume thatany loss of efficiency is due to a fluorine problem and attempt toadjust the fluorine to compensate, and thus do not compensate for theageing of the laser in any way.

It would be desirable to have a method that allows a laser to operatewithin acceptable parameters for a longer period of time using onlyinjects rather than refills to replenish the gas in the chambers, sothat the laser need not be shut off during the refill procedures. Itwould also be desirable to be able to compensate for the ageing of thelaser as part of such a method using only injects rather than refills.

SUMMARY OF THE INVENTION

Systems and methods for operating a dual chamber gas discharge laser,such as a MOPA excimer laser, using only injects rather than refillswhile compensating for the ageing of the laser by increasing thepressure in one or both of the chambers are disclosed.

In one embodiment, a dual chamber gas discharge laser light source isdescribed, comprising a master oscillator and an amplifier, each of themaster oscillator and amplifier having a laser chamber containing alasing medium gas comprising a halogen, and a gas replenishment systemincluding a controller executing a replenishment scheme at regularintervals, the replenishment scheme comprising injecting into theselected laser chamber at each inject opportunity a quantity of anon-halogen containing gas and a quantity of the halogen containing gasestimated to result in a desired amount of halogen gas in the chamberafter the inject opportunity; and after each N injects, where N is apredetermined number, adjusting the pressure in the chamber tocompensate for any change in the efficiency of the laser due to thelength of operation of the laser.

In another embodiment, a method of replenishing gas in a dual chambergas discharge laser light source having a master oscillator and anamplifier is described, each of the master oscillator and amplifierhaving a laser chamber containing a lasing medium gas comprising ahalogen, the method comprising the steps of: selecting a plurality ofinject opportunities occurring at regular intervals; injecting into theselected laser chamber at each inject opportunity a quantity of anon-halogen containing gas and a quantity of the halogen containing gasestimated to result in a desired amount of halogen gas in the chamberafter the inject opportunity; and after each N injects, where N is apredetermined number, adjusting the pressure in the chamber tocompensate for any change in the efficiency of the laser due to thelength of operation of the laser.

Still another embodiment discloses a non-transitory computer-readablemedium having embodied thereon a program, the program being executableby a processor to perform a method of automatically replenishing the gasin a laser chamber of a dual chamber gas discharge laser light sourcehaving a master oscillator and an amplifier, each of the masteroscillator and amplifier having a laser chamber containing a lasingmedium gas comprising a halogen, the method comprising the steps of:selecting a plurality of inject opportunities occurring at regularintervals; injecting into the selected laser chamber at each injectopportunity a quantity of a non-halogen containing gas and a quantity ofthe halogen containing gas estimated to result in a desired amount ofhalogen gas in the chamber after the inject opportunity; measuring anoperating parameter of the selected laser chamber that is indicative ofthe efficiency of the laser during each injection of gas into thechamber; estimating the ratio of the change of the measured operatingparameter to the number of shots fired by the laser; measuring thechange in pressure during each inject opportunity; determining the ratioof the change of the measured operating parameter to the change inpressure in the chamber; and after each N injects: estimating the changein the operating parameter over the N injects; estimating the pressurein the chamber that is appropriate to reverse the change in theoperating parameter over the N injects; and adjusting the pressure inthe chamber to the estimated pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified block diagram of a gas replenishment system100 for a dual chamber gas laser, such as a MOPA excimer laser,according to one embodiment.

FIG. 2 shows the relationship between the pressure in a laser chamber ofa dual chamber gas laser, such as a MOPA excimer laser, and thereciprocal of efficiency at different points in the life of the laser.

FIG. 3A illustrates the typical effect of continued shots and refills onthe efficiency of a dual chamber gas laser, such as a MOPA excimerlaser.

FIG. 3B illustrates a desired effect of maintaining the efficiency of adual chamber gas laser, such as a MOPA excimer laser, as shown in FIG.2A but without the use of refills of the gas in the laser chambers.

FIG. 4 shows the relationship between a series of injects and theresulting chamber pressure in a laser chamber of a dual chamber gaslaser, such as a MOPA excimer laser, according to one embodiment.

FIG. 5 is a graph of a laser parameter having a linear relationship tothe length of operation of the laser.

FIG. 6 shows a single inject in a in a laser chamber of a dual chambergas laser, such as a MOPA excimer laser, and the measurement of thesensitivity of a parameter of laser operation to a change in pressure,according to one embodiment.

FIG. 7 is a simplified flowchart showing one embodiment of the gasreplenishment method described herein.

DETAILED DESCRIPTION OF THE INVENTION

The present application describes a method and system for performinginjects of halogen gas into the chambers of a two chamber gas dischargelaser such as a MOPA excimer laser for the purpose of allowing operationof the laser within acceptable parameters without the necessity ofperforming refills. The method also allows for automatic compensationfor the ageing of the laser by increasing the pressure in the chambersover time. While manual adjustments to pressure have been madepreviously, it is believed that such automatic and substantiallycontinuous gas optimization without the need of refills has notpreviously been suggested or attempted in the prior art.

It is expected that an inject process as described herein will result inan increased period of operation of the laser without needing to suspendoperation during refills, as well as greater efficiency due to thecompensation for ageing of the laser. A parameter indicative of theperformance of the laser is measured, and the change in the parameterwith respect to the length of operation of the laser is estimated todetermine the ageing rate of the laser. The change in the parameter withrespect to the pressure in one of the chambers is also measured todetermine the change in efficiency due to pressure. At a selected time,the total change in the value of the parameter is estimated, indictingthe decrease in performance of the laser, and from this the amount ofchange in pressure that is needed to reverse the change in the value ofthe parameter is calculated. The pressure in the chamber is thenchanged, thus improving the performance of the laser to correct for theamount of time that the laser has been in operation.

There are a number of parameters indicative of the laser performancethat may be selected. In many cases, laser efficiency may be the easiestparameter to use. However, other parameters may be selected if desired;for example, when considering injects to the power amplifier chamber, Vmay be the discharge voltage, while in the case of the master oscillatorchamber V may be the delay time (“dtMOPA”) between an electricaldischarge in the master oscillator chamber that creates a laser shot andthe subsequent electrical discharge in the power amplifier chamber thatamplifies the shot

Still other parameters may be used, such as bandwidth, which may bemeasured by the integral of a certain percentage of the energy containedon either side of a center wavelength of a spectrum centered on thecenter wavelength. One bandwidth measure used in other contexts is theintegral of 95% of the energy is common and is known as E95% or simplyE95. Another parameter that may be used is the common voltage applied toboth chambers, for example, at the peaking capacitor of a compressionhead of a solid state pulsed power system (SSPPM) for each chamber, andthe energy output of one of the chambers. For the master oscillatorchamber this is designated as E_(MO). Other suitable parameters and/orcombinations of parameters for estimating the amount of, or rate ofconsumption of, fluorine will be apparent to those of skill in the art.

As stated previously, in order to obtain the full benefit of the methoddescribed herein, it is desirable to have a method of controlling thefluorine level more accurately than has been done previously. Many priorart methods of fluorine control are actually biased by the long termageing effects, i.e., they confuse normal fluorine depletion withageing, and thus cannot decouple accurate fluorine control from theageing effect. One such method of decoupling fluorine control fromageing effects is described in U.S. patent application Ser. No.11/094,313, commonly owned by the assignee of the present application.

A simplified block diagram of a gas replenishment system 100 for a dualchamber gas laser, such as a MOPA excimer laser, is shown in FIG. 1. TheMOPA excimer laser has a master oscillator 102 containing a laserchamber, and a power amplifier 104 also containing a laser chamber. Inoperation, the master oscillator 102 produces a first laser beam 106which is passed to the power amplifier 104 where it is amplified, toproduce an amplified laser beam 108 which is output to a scanner machine(not shown) for use in lithography.

Each laser chamber contains a mixture of gases; for example, in a givenexcimer laser each laser chamber might contain a halogen, e.g.,fluorine, along with other gases such argon, neon, (commonly known asrare gases) and possibly others in different partial pressures that addup to a total pressure P. For simplicity, the halogen gas is hereafterdescribed as fluorine, although the principles described herein may beapplied to other halogen gases as well.

Gas bottles 110 and 112 are connected to the master oscillator 102 andpower amplifier 104 through valves 114 to allow for replenishment of thegas in the laser chambers when desired. Gas bottle 110 typically mightcontain a mixture of gases including fluorine, argon and neon, known asan “M1 mix,” “tri-mix,” or often simply “fluorine,” while gas bottle 112might contain a mixture of argon, neon and/or other gases, but nofluorine, known as an “M2 mix,” “bi-mix,” or “rare gas.” A controller116, such as a processor or logic circuit, operates the valves 114 totransfer gases from bottles 110 and 112 into the laser chambers of themaster oscillator 102 and power amplifier 104 based upon certain data asdescribed further herein.

As is known in the art, two bottles of gas are needed, since thefluorine in gas bottle 110 is at a particular concentration that istypically higher than that desired for laser operation. In order to addthe fluorine to the laser chamber of the master oscillator 102 or poweramplifier 104 at a desired lower concentration, the gas in bottle 110must be diluted, and the non-halogen containing gas in bottle 112 isused for this purpose.

Although not shown, valves 114 typically include two valves for eachlaser chamber, an “injection” valve that allows gas to pass into and outof each chamber at a first rate, and a “chamber fill” valve that allowsgas to pass into and out of each chamber at a second, and faster, rate.In addition, the laser chambers in the master oscillator 102 and poweramplifier 104 contain blowers for mixing the gases that are in thechambers so that a homogenous mixture is maintained during operation.The blowers commonly also add heat to the gas.

As mentioned above, fluorine is consumed during operation of the laser.The resulting decrease in fluorine concentration typically causes a risein the discharge voltage required to produce a laser pulse. In addition,changes in fluorine concentration also affect the delay time between theelectrical discharges that cause production of the first laser beam 106and the amplified laser beam 108; this delay time is often referred toas “dtMOPA.”

Thus, the fluorine concentration must be replenished to keep the laseroperating within desired parameters. Further, a satisfactoryconcentration of fluorine must be maintained while keeping the gascontent in each laser chamber at a fixed pressure. Again, this issometimes done by injects, i.e., partial replenishment of the gas in thechamber, rather than a full refill in which the chamber is purged andthe gas completely replaced.

As with full refills, injects are typically done at fixed intervals,determined either by elapsed time between injects, or by the number of“shots,” i.e., pulses of the laser, that have been generated. In someprior art systems, injects are done in each chamber after approximatelyevery 1 million pulses by that chamber. For easier operation, theinjects to the laser chambers are staggered, so that while each chamberreceives an inject after about each million pulses, the power amplifier104 receives an inject approximately 500,000 pulses after the masteroscillator 102 receives an inject, and vice versa. Such timing ofinjects is described, for example, in U.S. Pat. No. 7,835,414, owned bythe assignee of the present application.

While a full refill simply replaces all of the gas in the laser chamber,an inject is intended mainly to replace the amount of fluorine that hasbeen consumed since the last refill or inject. Since it is mostly thefluorine that is consumed during operation, it is known in the prior artthat injects to the laser chambers in both the master oscillator andpower amplifier will include a fixed amount of the M2 mix, whichcontains no fluorine, and an amount of M1 mix containing enough fluorineto raise the concentration of fluorine in the chamber back to a desiredlevel, thus replacing the fluorine which has been consumed.

However, as previously performed in the prior art, injects do notcompletely restore the operating condition of the laser to its peak ordesired efficiency. Rather, the efficiency of the laser decreases overtime, even with injects; this is believed to be primarily due to thedegradation of optical elements from exposure to the laser pulses, butmay also be due to other physics of laser operation.

FIG. 2 shows an example of the relationship between the pressure in alaser chamber and the reciprocal of efficiency at different points inthe life of the laser.

Because of the reduction of laser efficiency over time, a curve showingthat relationship between efficiency and pressure, such as those shownin FIG. 2, typically moves up over time. Thus, a new laser might operatealong the curve labeled 201. After some number of shots, the laser mightoperate with less efficiency along the curve labeled 202, and afterstill more shots the laser might operate along the curve labeled 203.

As shown in FIG. 2, in general the efficiency of the laser is higher(and thus the reciprocal of the efficiency is lower) at greater gaspressures, although this is only true up to the maximum chamber pressureat which the laser can operate. (There is also a minimum requiredpressure for the laser. Different lasers will have different maximum andminimum pressures.)

If it is desired to operate the laser at a particular target efficiencyE_(T), while a new laser operating along curve 201 can achieve this at apressure P₁, an aged laser that operates along curve 202 will require ahigher pressure P₂ to achieve the same efficiency. Similarly, an evenolder laser operating along curve 203 will require a still higherpressure P₃ to achieve the target efficiency, again assuming that P₃ iswithin the maximum operating pressure of the laser.

Thus, as shown in the example of FIG. 2, it can also be seen that thedecrease in efficiency can be compensated for by increasing the pressurein the chamber, as long as the pressure remains less than the maximumchamber pressure. Thus, in one embodiment, when the efficiency of thelaser drops below some minimum desired efficiency, the pressure in thechamber is increased until the efficiency rises to an acceptable level.

It is because of the maximum (and minimum) chamber pressure thataccurate fluorine control is desirable for use with the method describedherein. In the absence of good control of the fluorine, increasing thepressure in the chamber may result in the maximum (or minimum) pressurebeing reached more quickly, thus reducing the time over which the lasercan operate with only injects.

FIG. 3A illustrates the typical effect of continued shots and refills onthe efficiency of a laser. The horizontal axis represents the shotsfired by the laser, and the vertical axis the reciprocal of theefficiency of the laser.

It is assumed here that upon startup the laser is optimized to operateat some target efficiency E_(T). The efficiency of the laser typicallydrops over time as shots are fired, and thus the reciprocal ofefficiency increases as shown.

After some number N of injects, even with the replenishment of fluorinefrom the injects the efficiency of the laser has dropped to some minimumdesired efficiency E_(M). In the absence of some change from the knownmethods of performing injects, the efficiency of the laser will continueto decrease.

In the prior art, this problem is avoided by replacing all of the gas inthe chamber by performing a refill in order to keep the laser operatingwithin the desired range of efficiency. Thus, in FIG. 2A, after each Ninjects a refill is performed, as shown by points 1, 2, 3, 4, and 5 onthe horizontal axis. At the conclusion of each refill, the gas may beoptimized, including adjustment of the pressure, until the efficiency ofthe laser is restored to a desired efficiency, here shown as the targetefficiency E_(T), and the efficiency then again decreases as the lasercontinues to fire shots. An automated process of gas optimization aftera refill is shown in U.S. patent application Ser. No. 13/174,640, filedJun. 30, 2011, and assigned to the assignee of the present application.

As explained previously, operation of the laser must stop during therefill process. Thus, it would be desirable to be able to restore theefficiency of the laser to its target efficiency E_(T) (or someefficiency close to that) using only an inject of some type rather thana refill. FIG. 3B illustrates what a plot of the reciprocal ofefficiency versus the number of shots fired might look like in such acase. Now there are no refills, but some other process that occursperiodically, for example after each M injects, and which would be ableto at least substantially restore the efficiency of the laser withoutthe need for a refill. (Note that M will typically be a different numberthan N as explained below.)

The increase in pressure in the chamber is obtained by reducing the sizeof the bleed at the end of an inject so that the pressure in the chamberafter the inject is completed is at the higher level necessary toincrease the efficiency of the laser as desired. After such an injectwith a smaller bleed, injects are resumed in the normal fashion, withnormal bleeds that reduce the pressure in the chamber to the new higherlevel at the end of each inject, until the efficiency of the laser hasagain dropped below the minimum desired efficiency.

This process may be seen in FIG. 4, which shows a series of injects andthe resulting chamber pressure according to one embodiment. It isassumed that the injects begin after a refill of the laser chambers andoptimization of the gas as described above. One of skill in the art willrecognize that a typical inject actually includes three steps, as seenin more detail in FIG. 6: first, M1 mix is added to the chamber,followed by the addition of M2 mix to the chamber, both of whichincrease the pressure in the chamber, and finally the mixed gas is bledfrom the chamber to reduce the increased pressure back down to thedesired pressure.

As seen in FIG. 4, after a refill is completed at point 1, the laserbegins operating with a chamber pressure of P₁, and injects areperformed at regular intervals as is known in the prior art. After someperiod of M injects, at point 2 the efficiency of the laser will fall tosome minimum desired efficiency, as seen in FIG. 3B. The typicalresponse to this in the prior art would be to purge the chamber of gas,refill it, and then optimize the gas as described above.

However, as stated above, in one embodiment of the present method,instead of a refill, when gas has been added to the chamber in the nextinject M+1, the amount of gas bled from the chamber is reduced so thatthe pressure in the chamber does not drop back to P₁ as in the priorinjects. As a result, at the end of this inject, the pressure in thelaser chamber is now at a pressure P₂, which is higher than the previouspressure P₁. Injects are then performed normally to replenish thefluorine in the chamber and keep the chamber pressure at the increasedpressure P₂, rather than the prior pressure P₁, as the laser continuesto operate.

After another M injects, the efficiency of the laser will have fallenagain. As was done after the first M injects, at the end of the nextinject, inject 2M+1, the amount of gas bled from the chamber will againbe reduced from the prior injects so that the pressure does not dropback to P₂, again resulting in an increase of pressure in the chamber toa new pressure P₃ which is higher than P₂, and thus again leading to anincrease in the efficiency of the laser.

Thus, over the first M injects the pressure in the chambers will be P₁as shown in FIG. 2, and over this period the efficiency of the laserwill drop from its starting position of the target efficiency E_(T)toward a minimum efficiency E_(M) as shown in FIG. 3B. At the pointlabeled 2 on FIG. 3B, inject M+1 raises the pressure in the chambersfrom P₁ to P₂ as shown on FIG. 2, and restores the efficiency to E_(T)as shown at point 1 in FIG. 3B. Similarly, over the next M injects thepressure in the chambers will be P₂ (FIG. 2), and again the efficiencywill drop as shots are fired (FIG. 3B). At point 3 on FIG. 3B, inject2M+1 will again raise the pressure in the chambers, now from P₂ to P₃,and again return the efficiency to the target efficiency E_(T).

The values of M and N are somewhat arbitrary and will depend on theparticular laser and the choices made by a laser operator or engineer.In some lasers, refills may be done approximately every 2 billionpulses, and injects approximately every million pulses, so that theremay be about 2,000 injects in between refills (i.e., N=2000). In someembodiments of the present method, it is believed to be preferable toadjust the pressure as described more often than every 2,000 injects, inorder to keep the efficiency of the laser closer to its optimal valuethan typically occurs between refills. It is believed that adjusting thepressure in the range of every 50 to 200 injects (i.e., 50≦M≦200) mayresult in smaller variations in the efficiency of the laser.

It may thus be seen that in principle it is possible to compensate forthe aging of the laser by increasing the pressure in the chambers inthis fashion. The question is thus how to determine how much to increasethe pressure in order to obtain the proper compensation for the age ofthe laser.

In one embodiment, it is first determined how fast the efficiency of thelaser deteriorates, and thus the relationship between the age of thelaser and the change in the efficiency curves shown on FIG. 3, i.e., howmany shots it takes for the laser's efficiency curve to move, forexample, from curve 201 to curve 202, and from curve 202 to curve 203.This is considered the “aging rate” of the laser.

Next, to determine the ageing rate, first a parameter V is selected andmeasured. As above, while efficiency may be the easiest parameter toconsider, other parameters may be used as well. Again, in oneembodiment, when considering injects to the power amplifier chamber, Vmay be the discharge voltage, while in the case of the master oscillatorchamber V it may be the delay time dtMOPA. In other embodiments, V maybe some other measurement such as bandwidth, for example E95, the commonvoltage applied to both chambers, for example, at the peaking capacitorof a compression head of a solid state pulsed power system (SSPPM) foreach chamber, or the energy output of one of the chambers such as E_(MO)for the master oscillator chamber. Other suitable parameters and/orcombinations of parameters for estimating the amount of, or rate ofconsumption of, fluorine will be apparent to those of skill in the art.

The value of V in general will be related to the amount of fluorine aswell as the age of the laser. However, if the amount of fluorine can beaccurately controlled and remains constant, then any change in the valueof V (ΔV or dV) will only be due to the effect of ageing, and thus goodfluorine control is desirable to obtain the maximum benefit of thedescribed method. It is assumed hereafter that such control is presentso that the amount of fluorine does remain approximately constant, sothat the change of V is solely due to the ageing, i.e., the firing ofshots, of the laser.

With approximately constant fluorine, measuring the change in V over anumber of shots of the laser (Δt or dt) results in a value dV/dt, theestimation of which will be apparent to one of skill in the art. Oneexample of accurate fluorine control and the estimation of dV/dt isshown in U.S. patent application Ser. No. 13/251,181, filed on Sep. 30,2011, and assigned to the assignee of the present application, which isincorporated herein by reference. Other methods of fluorine control andthe estimation of dV/dt are present in the prior art.

In the simplest example, the value of dV/dt will be a constant, so thatthe value of V over time is simply a straight line having some slope, asshown in FIG. 5. As shown here, after M injects, the value of V changesfrom V₁ at time T₁ to V₂ at time T₂. (Injects typically occur at fixedintervals based on the number of shots of the laser.) To compensate forthis change in V due to the ageing of the laser, it is desired to returnthe value of V to V₁.

Next, the change in pressure required to return the value of V to V₁ isdetermined. The sensitivity of the parameter V to pressure, i.e., dV/dP,is first determined. In one embodiment, this is done by comparing thevalue of the signal V to the pressure in the chamber at certain pointsduring the inject process as shown in FIG. 6. FIG. 6 shows a singleinject, which includes the injection of M1 mix and the subsequentinjection of M2 mix, which raises the pressure in the chamber to P_(X),and then a bleed of the chamber to reduce the pressure to the desiredpressure, here designated P_(Y).

A value of V designated V_(X) is measured at the time when the pressureis P_(X), i.e., after the M1 and M2 mix have been injected, and againwhen the pressure is P_(Y) (designated V_(Y)), i.e., after gas has beenbled from the chamber to reduce the pressure to the desired pressure.The difference in the signal V is taken by calculating V_(X)−V_(Y), asis the difference in pressure P_(X)−P_(Y). The value of dV/dP is thengiven by:

$\frac{V}{P} = \frac{V_{X} - V_{Y}}{P_{X} - P_{Y}}$

Note that while FIG. 6 shows the value of V decreasing with increasingpressure, the opposite may be true, depending upon which particularparameter is selected as V.

As above, after M injects, the change in V is known or estimated (i.e.,dV/dt times t, where t is again the number of shots fired), as shown inFIG. 5. If dV/dP is approximately constant during the period of the Minjects, in the simplest approximation, the appropriate change inpressure that will compensate for the change in V is easily determinedby:

${V_{2} - V_{1}} = {\frac{V}{P}\left( {P_{2} - P_{1}} \right)}$

where V₁ and V₂ are the values of V as measured at time T₁ and T₂ asabove, and P₁ is the pressure at time T₁, and P₂ is the desired pressureat time T₂.

However, it is likely that dV/dP is not constant during the period, andthus in some embodiments, a more detailed determination of the value ofdV/dP after M injects is desirable. In one embodiment, this may beaccomplished by calculating dV/dP after each inject as shown in FIG. 6,and filtering the resulting values of dV/dP through a filter, forexample, a low pass filter.

Thus, every M injects, another estimate of the current value of dV/dP isobtained from the values of dV/dP over the most recent M injects to getthe new desired pressure. In general, as seen in FIG. 3, the describedmethod will generally result in increasing the pressure in the chamberto compensate for the ageing of the laser. However, in some rarecircumstances, it may be appropriate to reduce the pressure in thechamber. In such a case, the bleed of the next inject is simply extendeduntil the pressure drops to the desired level.

In some cases, the value of dV/dP may vary very quickly, and the use ofa low pass filter will tend to smooth out the variations and eliminatenoise. It will be appreciated that such a low pass filter may alsoresult in an estimate of dV/dP at a given moment that differs from thevalue of dV/dP calculated for the last inject. If the bandwidth of thefilter is very small, most of the noise in the value of dV/dP will beeliminated, but if there is a large change in the value of dV/dP, thelag time for the output of the filter to catch up to the actual value ofdV/dP is increased. One of skill in the art will appreciate how toselect a filter to obtain a desired balance of noise reduction andconvergence of the output value.

In some embodiments, it may be desirable to change the pressure byslightly less than what is calculated, i.e., to remove slightly lessthan the entire change in V, in order to allow for noise in themeasurements.

The method described herein can be used with both chambers of a dualchamber laser. However, it will typically be desirable to use adifferent parameter V for each chamber, and the resulting pressures forefficient operation may be different for each chamber. For example, forthe power amplifier laser chamber it may be appropriate to use thedischarge voltage as the parameter V, while for the master oscillatorchamber it may be preferable to use dtMOPA, the laser bandwidth, or E95.Alternatively, the injects to the PA chamber may remain fixed asdescribed in co-pending application Ser. No. 13/098,259, owned by theassignee of the current application, and the method described hereinused only for the MO chamber.

Where a different parameter V is used for each chamber, the describedcalculations become somewhat more complicated due to the couplingeffects between the chambers. While these effects are typically small,they are easily taken into account by the use of matrix inversiontechniques that will be familiar to those of skill in the art.

FIG. 7 is a simplified flowchart showing one embodiment of the gasreplenishment method described herein. As above, it is assumed that thelaser chambers have been filled, and the gas optimized. At step 701, itis determined what inject intervals have been selected. As above, thesemay have been selected either by elapsed time or number of shots fired.

At step 702, the initial pressure P1 is determined, and dV/dt isestimated. Alternatively, dV/dt may be determined by measurement asexplained herein as the injects occur.

For each of the first M−1 injects, gas is injected into the chamber, anddV/dP for the inject is calculated at step 703.

At step 704, for the Mth inject, gas is injected, dV/dP for the injectis calculated, and the change in pressure ΔP appropriate to compensatefor the change in V over the N injects is calculated.

At step 705, during inject M+1, after the gases have been injected, thechamber is bled until the new desired pressure P2, i.e., P1+ΔP, isreached. Inject M+1 is also treated as the first inject for the nextperiod of M injects, and the process repeats as long as the laser isoperated, and the pressure stays within the minimum and maximum pressureat which the laser will operate.

In some embodiments, upon the commencement of laser operation after arefill or a significant pause in operation, for example overapproximately an hour, it may be advantageous to wait for a small numberof injects before performing the method of FIG. 7, in order to let anythermal transients that may arise die out and arrive at a steady stateof operation. Thus, for example, it may be determined that for the firstfew injects, on the order of 5 to 10, dV/dt is not calculated, and thatfor a few more injects, on the order of 10 to 20, dV/dt is calculatedbut will not be used if the value appears to vary excessively. This mayallow for a more accurate calculation of dV/dt, and thus for moreaccurate control of the laser efficiency with the described method.

The disclosed system and method has been explained above with referenceto several embodiments. Other embodiments will be apparent to thoseskilled in the art in light of this disclosure. Certain aspects of thedescribed method and apparatus may readily be implemented usingconfigurations or steps other than those described in the embodimentsabove, or in conjunction with elements other than or in addition tothose described above.

For example, it will be understood by those skilled in the art thatwhile the preferred embodiment is a master oscillator-power amplifiermulti-chambered excimer or molecular fluorine gas discharge laser system(“MOPA”) the system may also be configured to have otheroscillator/amplifier configurations, such as a master oscillator-poweroscillator (“MOPO”), a power oscillator-power amplifier (“POPA”) or apower oscillator-power oscillator (“POPO”) configuration, or the like.It will also be understood that in each of such configurations theoutput of the first oscillator stage is amplified in some fashion in thesecond stage, regardless of whether the second stage is a poweramplifier or a power oscillator.

Similarly, unless otherwise indicated specifically to the contraryreference to a master oscillator stage or chamber (“MO”) in theSpecification or the appended claims, and/or a power amplifier stage orchamber (“PA”) in the Specification or appended claims, shall beconsidered to be broad enough to cover any oscillator first stage orchamber feeding an output into any amplifier second stage or chamber foramplification, and the term oscillator chamber or oscillator stage isbroad enough to cover any such oscillator stage and the term amplifierchamber or stage is broad enough to cover any such amplifier stage.Further, while the above description uses a two stage or chamber laseras an example, the system and method disclosed might also be applied toa single chamber laser or any multi-chamber laser.

It should also be appreciated that the described method and apparatuscan be implemented in numerous ways, including as a process, anapparatus, or a system. The methods described herein may be implementedby program instructions for instructing a processor to perform suchmethods, and such instructions recorded on a computer readable storagemedium such as a hard disk drive, floppy disk, optical disc such as acompact disc (CD) or digital versatile disc (DVD), flash memory, etc.The methods may also be incorporated into hard-wired logic if desired.It should be noted that the order of the steps of the methods describedherein may be altered and still be within the scope of the disclosure.

These and other variations upon the embodiments are intended to becovered by the present disclosure, which is limited only by the appendedclaims.

What is claimed is:
 1. A dual chamber gas discharge laser light source,comprising: a master oscillator having a laser chamber containing alasing medium gas comprising a halogen; an amplifier having a laserchamber containing a lasing medium gas comprising a halogen; and a gasreplenishment system including a controller configured to perform areplenishment scheme in a laser chamber at inject opportunitiesoccurring at regular intervals, the replenishment scheme comprising:injecting into the selected laser chamber at each inject opportunity aquantity of a non-halogen containing gas and a quantity of the halogencontaining gas estimated to result in a desired amount of halogen gas inthe chamber after the inject opportunity; and after each M injects,where M is a predetermined number, adjusting the pressure in the chamberto compensate for any change in the efficiency of the laser due to thelength of operation of the laser.
 2. The dual chamber gas dischargelaser light source of claim 1 wherein adjusting the pressure in thechamber to compensate for any change in the efficiency of the lasercomprises: measuring an operating parameter of the selected laserchamber that is indicative of the efficiency of the laser during eachinjection of gas into the chamber; estimating the ratio of the change ofthe measured operating parameter to the number of shots fired by thelaser; measuring the change in pressure during each inject opportunity;determining the ratio of the change of the measured operating parameterto the change in pressure in the chamber; after each M injects:estimating the change in the operating parameter over the M injects; andestimating the pressure in the chamber that is appropriate to reversethe change in the operating parameter over the M injects; and adjustingthe pressure in the chamber to the estimated pressure.
 3. The dualchamber gas discharge laser light source of claim 1 wherein the halogencomprises fluorine.
 4. The dual chamber gas discharge laser light sourceof claim 1 wherein the regular intervals for inject opportunities aredetermined by factors comprising one or both of elapsed time and shotcount.
 5. The dual chamber gas discharge laser light source of claim 2wherein the selected laser chamber is the amplifier laser chamber andthe operating parameter is the discharge voltage in the amplifier laserchamber.
 6. The dual chamber gas discharge laser light source of claim 2wherein the selected laser chamber is the master oscillator laserchamber and the operating parameter is the discharge timing differentialbetween the master oscillator and amplifier.
 7. The dual chamber gasdischarge laser light source of claim 2 wherein the selected laserchamber is the master oscillator laser chamber and the operatingparameter is the bandwidth of the laser light source.
 8. The dualchamber gas discharge laser light source of claim 2 wherein the selectedlaser chamber is the master oscillator laser chamber and the operatingparameter is E95.
 9. The dual chamber gas discharge laser light sourceof claim 1 wherein the value of M is between approximately 50 andapproximately
 200. 10. The dual chamber gas discharge laser light sourceof claim 1 wherein modeling the amount of halogen gas in the chamberafter an inject opportunity further comprises modeling the amount ofhalogen gas in the chamber at a selected point after the injectopportunity and before the immediately subsequent inject opportunity.11. A method of replenishing gas in a dual chamber gas discharge laserlight source having a master oscillator and an amplifier, each of themaster oscillator and amplifier having a laser chamber containing alasing medium gas comprising a halogen, the method comprising the stepsof: selecting a plurality of inject opportunities occurring at regularintervals; injecting into the selected laser chamber at each injectopportunity a quantity of a non-halogen containing gas and a quantity ofthe halogen containing gas estimated to result in a desired amount ofhalogen gas in the chamber after the inject opportunity; and after eachM injects, where M is a predetermined number, adjusting the pressure inthe chamber to compensate for any change in the efficiency of the laserdue to the length of operation of the laser.
 12. The method ofreplenishing gas of claim 11 wherein adjusting the pressure in thechamber to compensate for any change in the efficiency of the lasercomprises: measuring an operating parameter of the selected laserchamber that is indicative of the efficiency of the laser during eachinjection of gas into the chamber; estimating the ratio of the change ofthe measured operating parameter to the number of shots fired by thelaser; measuring the change in pressure during each inject opportunity;determining the ratio of the change of the measured operating parameterto the change in pressure in the chamber; after each M injects:estimating the change in the operating parameter over the M injects;estimating the pressure in the chamber that is appropriate to reversethe change in the operating parameter over the M injects; and adjustingthe pressure in the chamber to the estimated pressure.
 13. The method ofreplenishing gas of claim 11 wherein the halogen comprises fluorine. 14.The method of replenishing gas of claim 12 wherein selecting theplurality of inject opportunities further comprises selecting theregular intervals based upon factors comprising one or both of elapsedtime and shot count.
 15. The method of replenishing gas of claim 12wherein the selected laser chamber is the amplifier laser chamber andthe operating parameter is the discharge voltage in the amplifier laserchamber.
 16. The method of replenishing gas of claim 12 wherein theselected laser chamber is the master oscillator laser chamber and theoperating parameter is the discharge timing differential between themaster oscillator and amplifier.
 17. The method of replenishing gas ofclaim 12 wherein the selected laser chamber is the master oscillatorlaser chamber and the operating parameter is the bandwidth of theexcimer laser light source.
 18. The method of replenishing gas of claim12 wherein the selected laser chamber is the master oscillator laserchamber and the operating parameter is E95.
 19. The method ofreplenishing gas of claim 12 wherein the value of M is betweenapproximately 50 and approximately
 200. 20. The method of replenishinggas of claim 12 wherein modeling the amount of halogen gas in thechamber after an inject opportunity further comprises modeling theamount of halogen gas in the chamber at a selected point after theinject opportunity and before the immediately subsequent injectopportunity.
 21. A non-transitory computer-readable medium havingembodied thereon a program, the program being executable by a processorto perform a method of replenishing gas in a dual chamber gas dischargelaser light source having a master oscillator and an amplifier, each ofthe master oscillator and amplifier having a laser chamber containing alasing medium gas comprising a halogen, the method comprising the stepsof: selecting a plurality of inject opportunities occurring at regularintervals; injecting into the selected laser chamber at each injectopportunity a quantity of a non-halogen containing gas and a quantity ofthe halogen containing gas estimated to result in a desired amount ofhalogen gas in the chamber after the inject opportunity; measuring anoperating parameter of the selected laser chamber that is indicative ofthe efficiency of the laser during each injection of gas into thechamber; estimating the ratio of the change of the measured operatingparameter to the number of shots fired by the laser; measuring thechange in pressure during each inject opportunity; determining the ratioof the change of the measured operating parameter to the change inpressure in the chamber; and after each M injects: estimating the changein the operating parameter over the M injects; estimating the pressurein the chamber that is appropriate to reverse the change in theoperating parameter over the M injects; and adjusting the pressure inthe chamber to the estimated pressure.